and a method for optical resolution of the optical isomers thereof, and also an optically active substance thereof.
By hydrolyzing the optical isomers of these esters in the retention of the configurations, optically active 2-phenylpropionic acids can readily be obtained.
Optically active 2-phenylpropionic acid is a useful compound to be used widely as a resolving agent for bases, and it is also useful as an intermediate compound or modifying agent of pharmaceuticals, agricultural chemicals, etc.
In the prior art, as the method for preparation of optically active 2-phenylpropionic acid capable of optical resolution of a racemic modification, the following methods have been known:
(a) Diastereomer resolution method by salt formation with a basic resolving agent: for example, the method in which .alpha.-phenethylamine is used as the resolving agent [see Arkivkemi., 10, 283 (1956)], similarly the method in which .alpha.-phenyl-.beta.-(p-tolyl)ethylamine is used [see Japanese Unexamined Patent Publication No. 63946/1981];
(b) Diastereomer resolution method by ester formation with an optically active alcohol: for example, the method in which (-)-menthyl ester is obtained as an intermediate [see German OLS DE.3116474], the method in which .alpha.-phenethyl ester is obtained as an intermediate [see Angew. Chem. 93, 919 (1981)].
On the other hand, concerning the compounds of the present invention, both optical isomers and racemic modification have not been known in the art for .beta.-naphthyl ester and 2-(.beta.-naphthyl)ethyl ester of 2-phenylpropionic acid. Also, as for p-nitrophenyl ester of 2-phenylpropionic acid, although there are descriptions thereabout in Chemical Abstracts 89, 107223q (original report, Ciba Found. Symp., 53, 149 (1978)) and 84, 16437t (original report, J.C.S. Chem. Commun., 969 (1974)), there is no description about the physical properties and the method for preparation in any of these literatures. Thus, it may be said that these compounds has been isolated and purified for the first time in the present invention.
Any of the techniques for preparation of optically active 2-phenylpropionic acid by optical resolution of the prior art requires an expensive resolving agent. Also, when the useless optical isomer of 2-phenylpropionic acid is racemized for reuse, the method of (a) is unpractically too slow in racemization. On the other hand, according to the method of (b), in addition to the problem of no realization of complete racemization due to influence by the optically active alcoholic moiety, there is also involved the problem of side reaction which causes the optical purity of the alcoholic moiety to be lowered. Thus, both the methods (a) and (b) can hardly be said to be practical preparation methods.
To the contrary, the method by way of preferential crystallization requires no expensive resolving agent, and it is an advantageous method which enables optical resolution by crystallization operation employing a small amount of seed crystals. However, so far as 2-phenylpropionic acid is concerned, no compound for which this method is applicable has been known at all.